A conventional pump for liquid chromatography generally has a construction in which eluate is sucked and discharged by a plunger reciprocating in a cylinder and a construction has been known in which a first plunger and a second plunger are independently driven by a motor to reduce a pulsating flow rate by driving both of the plungers cooperatively. One example of this construction is disclosed, for example, in a patent document 1 described below.
In this conventional technology, while a first plunger reciprocates one time, a second plunger also reciprocates one time to correct a pulsating flow rate caused by the suction of the first plunger by the operation of the second plunger. That is, the first plunger determines a liquid flow rate and the second plunger is used for correcting the pulsating flow rate caused by the first plunger.
[Patent document 1] Japanese Patent Laid-Open No. S63(1988)-75375
However, in the above-described conventional pump for liquid chromatography, in a case where in order to feed liquid at an extremely low flow rate, the speed reduction ratio of a motor is increased to decrease the speed of the plunger or the diameter or stroke of the plunger is decreased, conversely, the liquid can not be sent at a large flow rate. For this reason, this pump presents a problem that it takes much time to fill eluate into the passage of a measurement system on the downstream side of the pump and a problem that bubbles remaining in the pump can not be easily discharged. When the bubbles can not be discharged, even if the plunger reciprocates, the plunger only compresses or expands the bubbles and cannot discharge the liquid. Therefore, there is presented a problem that this pump is not appropriate for constructing a pump used for an extremely low flow rate.